Despite massive international efforts to control Schistosomiasis, there is yet no effective control method for either the parasite or the snail and the number of infected people has continued to increase to the currently estimated 200 million. A few reports of the incidental infection of Schistosomatoid larvae in their snail hosts by microsporidans suggest that possibility that microsporidosis in trematodes may be employed as a control method. We, therefore, propose to evaluate the potential of microsporidans as biological control agents for Schistosoma mansoni in its snail host, Biomphalaria glabrata. If successful, this research could lead to the effective control of S. mansoni with a specific, self-perpetuating agent which does not leave a toxic residue. Nosema strigeoidae and a yet unnamed microsporidan, both available, will be employed as test hyperparasites for the control of S. mansoni. Other species will also be tested. The Experimental procedure will include: 1. Studying the specificities of microsporidans (infecting either larval trematodes in snails or host snails directly) in the naturally occurring system, in the B. glabrata-S. mansoni system, and preliminary studies on their effects in the environment. 2. Studying the route of invasion of microsporidans into snails while en route to the trematodes, and while gaining access into the trematodes. Both electron microscopy and cytochemistry will be employed. This information will be useful in helping to understand, and therefore promote, a successful system. 3. Studying the pathogenesis of the microsporidans on larval trematodes, including such parameters as the relationship between dosage studies and the percentage of shedding and cercarial infectivity.